How to make a procedural planet material with varied roughness of land and water in Cycles with Blender 2.8?
Take the factor node output of the noise texture and feed it into a color ramp object. Take the color output of the color ramp and feed it to roughness of the principled shader. Use the color ramp object to create a sharp gradient that cuts off at the same point as your land.  You can use the same color ramp to set a lot of different properties, granting you sophisticated control over land vs sea.

You can use a more complicated ramp to vary the land behavior vs altitude and other subtle shading effects.